In recent years the volume of information has grown rapidly. Utilization of electronic instruments has also increased significantly. As a result, personal computers, notebook computers, PDAs, and handsets are frequently used by the general public in their daily lives. For entering data into electronic instruments, keyboards or keypads are the most convenient input devices.
However, using a keyboard often causes physical injury. Therefore, these days many keyboards are more ergonomically designed. Still, there are many problems when users strike the button keys during operation. For instance, depending on the user's mood and physical conditions, the depressing force on the button keys of the keyboard could be too small or too large. If the depressing force is too small data cannot be input as desired. If the depressing force is too strong for a prolonged period of time, finger muscles are stressed and may be injured.
Therefore, it is necessary to alert people when the striking force on the button keys is excessive so that they can change the depressing force on the button keys to prevent muscle injury.